Apoptosis resistance is a hallmark of human cancer (1-3). Cancer cells must overcome a continual bombardment by cellular stresses, such as DNA damage, oncogene activation, aberrant cell cycle progression, and harsh microenvironments, that would cause normal cells to undergo apoptosis. One of the primary means by which cancer cells evade apoptosis is by up-regulation of anti-apoptotic proteins of the Bcl-2 family. Targeting key apoptosis regulators to overcome apoptosis-resistance and promote apoptosis of tumor cells is a new cancer therapeutic strategy (4,5).
Bcl-2 proteins function as critical regulators of apoptosis in both cancer and normal cells (6-10). Bcl-2 proteins serve as a check on apoptosis allowing healthy and useful cells to survive. This protein family includes anti-apoptotic proteins, such as Bcl-2, Bcl-xL, and Mcl-1, and pro-apoptotic molecules, including Bid, Bim, Bad, Bak and Bax (6-10). While normal cells have low expression levels of the anti-apoptotic Bcl-2 and Bcl-xL proteins, these proteins are found to be highly overexpressed in many different types of human tumors.6-10 This overexpression has been linked to poor prognosis in several types of cancer, and to clinical resistance to chemotherapeutic agents and radiation (6-10). Consistent with clinical observations, laboratory studies have established that overexpression of Bcl-2 or Bcl-xL causes cancer cells to become more resistant to chemotherapeutic agents in vitro and in vivo (6-10). Inhibition of apoptosis by Bcl-2 contributes to cancer by inhibiting cell death. Therefore, targeting Bcl-2 and/or Bcl-xL has been pursued as a cancer therapeutic strategy (11-34). Inhibiting Bcl-2 activity in cancer cells can reduce chemotherapeutic resistance and increase the killing of cancer cells.
Bcl-2 and Bcl-xL proteins inhibit apoptosis by heterodimerization with pro-apoptotic Bcl-2 family proteins, such as Bak, Bax, Bim, Bid, Puma, and Bad (6-10). Experimentally determined three-dimensional structures of Bcl-xL and Bcl-2 have shown that these proteins possess a well-defined groove, which interacts with the BH3 (Bcl-2 Homology 3) domain of the pro-apoptotic Bcl-2 proteins (38-42). It has been proposed that non-peptide small molecules designed to block the heterodimerization of Bcl-2/Bcl-xL proteins with their pro-death binding partners may be effective as antagonists of Bcl-2/Bcl-xL, and that such small molecule inhibitors may have a great therapeutic potential for the treatment of human cancers in which Bcl-2 and/or Bcl-xL are highly expressed (18-37).
Although non-peptide, small molecule inhibitors of Bcl-2/Bcl-xL have been reported, most of the inhibitors have weak to modest affinities for these proteins and lack a well-defined mode of action for their cellular activity (18-37). The exceptions are ABT-737, ABT-263, and their analogues (26-34). ABT-737 and ABT-263 bind to Bcl-2, Bcl-xL, and Bcl-w with very high affinities (Ki<1 nM) and have high specificity over Mcl-1 and A1, two other anti-apoptotic Bcl-2 proteins (26, 32, 34). ABT-263 has advanced into Phase I/II clinical trials and shows promising antitumor activity in the clinic (45).
Despite the discovery of ABT-737 and ABT-263, the design of potent, non-peptide inhibitors of Bcl-2/Bcl-xL remains a significant challenge in modern drug discovery. Accordingly, a need still exists in the art for Bcl-2/Bcl-xL inhibitors having physical and pharmacological properties that permit use of the inhibitors in therapeutic applications. The present invention provides compounds designed to bind to Bcl-2/Bcl-xL and inhibit Bcl-2/Bcl-xL activity.